(a) Field of the Invention
The present invention relates to an electrostatic ink jet recording head and, in particular, to an ink jet recording head assembly using a liquid ink containing charged toner particles in an insulating solvent.
(b) Description of the Related Art
In a conventional ink jet recording head wherein ink is used as a recording material, the technique for supplying the liquid ink is categorized in two techniques. In the first technique, the amount of ink dissipated from an ink chamber as a result of the ejection is replenished from an ink reservoir by utilizing a meniscus force formed at an ejecting portion of the head body without using a pump. In this technique, the ink flows from the ink reservoir to the ink chamber without circulation.
With the first technique as generally adopted in a drop-on-demand type, since the ink ejection depends on the action of the meniscus force, there is a problem that a sufficient amount of toner particles cannot be replenished into the solvent which fills the ink chamber.
The second technique utilizes a pump for supplying the ink from the ink reservoir to a head body. This technique is employed in an ink jet recording head of a continuous type described in Patent Publications No. JP-A-5(1993)-261,936 and JP-A-5(1993)-185,600, for example. In this technique, the liquid ink is driven, as by a pump, to flow in one direction from the ink reservoir to the ink chamber, and the ink entering the ink chamber forces the preceding ink which has been present in the ink chamber back to the ink reservoir, thereby achieving a circulation of the ink between the ink reservoir and ink chamber.
Recently, an electrostatic ink jet recording head is developed in which a record is produced by using an ink containing charged toner particles dispersed in an insulating solvent. With an ink jet recording head of this type, a drop-on-demand technique is generally used in which charged toner particles in the solvent are subject to an electric field to be ejected from the surface of the meniscus, thereby producing a record on a recording medium. While the ink droplet is accompanied by a minimal amount of solvent, it is substantially composed of a cluster of toner particles. On the other hand, during the time interval when the ejection of the ink does not take place, the solvent is retained within the ink chamber by the surface tension produced by the meniscus formed at the ink ejecting portion.
In an ink jet recording head of the type described above, the toner in the solvent continues to be dissipated as the toner ejection continues, and hence the concentration of the toner decreases gradually in the ink. As a result, the amount of toner concentrated or collected at the ink ejecting portion decreases, with the consequence of a reduction in the amount of toner ejection, which in turn results in a small diameter of printed dots and a reduction in the printing density. If the ejection is further continued, the recording operation is not effected even though the ink chamber is filled with the solvent. Accordingly, in the recording head of this type, it is necessary to provide a fresh ink from the ink reservoir to the ink chamber to maintain the toner concentration in the solvent.
In the second technique, the ink ejection depends on the driving pressure by the pump to produce an ink flow. Hence, the technique is inadequate for use with the electrostatic ink jet recording head because the ink meniscus formed in the ink ejecting portion cannot be maintained at a high ink pressure within the ink chamber, causing an ink leakage from the ejecting portion. Further, since the second technique requires a pump for providing the ink circulation, it is difficult to obtain a compact size of the device, a reduction in manufacturing costs and an easy maintenance.
It is an object of the invention to provide an electrostatic ink jet recording head of the drop-on-demand type, wherein a reduction in the amount of toner contained in the solvent is effectively replenished in the ink chamber to assure that a stabilized printing operation be maintained to produce an excellent record.
The present invention provides an electrostatic ink jet recording head comprising: a head body defining an ink chamber for receiving liquid ink containing charged toner particles, the ink chamber having an inlet port disposed in the vicinity of a front end of the ink chamber, an outlet port disposed in the vicinity of a rear end of the ink chamber, and an ink ejecting slit, extending along the front end of the ink chamber, for ejecting the ink therethrough; a set of electrodes for ejecting toner particles from the ink chamber, the set of electrodes including a plurality of ink ejection electrodes arranged within the ink chamber along the ink ejecting slit, an electrophoretic electrode disposed within the ink chamber and opposed to tips of the plurality of ink ejection electrodes with an intervention of the ink inlet port, and an opposing electrode disposed outside the ink chamber and opposed to the tips of the plurality of ink ejection electrodes; and an ink reservoir, disposed above the ink chamber and connected to the ink inlet port and outlet port, for providing the liquid ink by gravity to the ink chamber through the inlet port and for receiving the liquid ink from the ink chamber through the outlet port.
In accordance with an embodiment of the invention, a voltage of the same polarity as the charged toner particles dispersed in the liquid ink is applied to the electrophoretic electrode to migrate the toner particles by electrophoresis, whereby the toner particles are collected or concentrated in the ink ejecting portion of the head body to form convex ink meniscuses. When a designed voltage pulse of the same polarity as the toner particles is applied to at least one of the ejection electrodes, a cluster of toner is ejected from the ink chamber. The toner particles thus dissipated by the ejection are replenished at any time from the other portion of the ink chamber under the influence of an electric field provided by the electrophoretic electrode and from the ink reservoir through the ink inlet port.
Specifically in the present invention, the ink toner is replenished from the ink reservoir by gravity without a pump so that the power dissipation of the ink jet recording head is reduced while achieving a compact size of the head and a reduction in manufacturing costs, without the necessity of a pump maintenance.
It is preferable to select a flow resistance against the ink in the flow path from the ink outlet port to the ink reservoir be lower than the ink meniscus force in order to allow the ink in the chamber to return through the outlet port and to receive new liquid ink from the ink reservoir, substantially without an ink leakage from the ink ejecting slit.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.